In the field of non-impact printing, heat transfer recording is rapidly increasing, particularly using various terminal printers. In recent years, color hard copy has been increasingly needed, and a sublimation type heat transfer printer has been developed as a video printer.
Heat transfer recording can be generally divided into two types: melt type heat transfer recording and sublimation type heat transfer recording. The sheet for use in melt type heat transfer recording has a basic structure in which a colorant is compounded in waxes capable of melting on heating and coated on a support in a thickness of about 5 .mu.m. This sheet has the advantages that sensitivity is high and storage stability is good, but the disadvantages that gradation cannot be obtained unless specified techniques are employed, recording cannot be conducted repeatedly, and transferability to a rough surface is inferior. The sheet for use in sublimation type heat transfer recording has a basic structure in which a transfer layer composed mainly of sublimation dye and a binder is coated on a heat-resistant support. This sheet has the advantage that gradation can be easily obtained. However, the sheet has the serious disadvantages that a large amount of energy is needed for obtaining the necessary recording density and special dye-receiving paper is needed, so that ordinary paper cannot be used. Another disadvantage of this sheet is that the color of the formed image disappears with the lapse of time.
Utilization of microcapsules in a heat transfer sheet is known. For example, microcapsules containing a colorant are described in, for example, U.S. Pat. No. 4,564,534, Japanese patent application (OPI) Nos. 207286/83, 211498/83, and 85992/85 (the term "OPI" as used herein means an "unexamined published Japanese patent application"). Microcapsules containing a colorant and a foaming agent are described in, for example, U.S. Pat. No. 4,564,534, Japanese patent application (OPI) Nos. 59897/83 and 224790/83. In these microcapsules, the microcapsule walls are broken by application of pressure utilizing a platen, or gas pressure produced by the foaming agent, or heat pressure, and the colorant contained in the microcapsules is released.
In these microcapsules, therefore, it is necessary to increase the amount of enegy applied or the pressure applied.
Another technique is to control the glass transition temperature of the microcapsule wall to the range of 0.degree. to 120.degree. C. (as described in Japanese patent application (OPI) Nos. 189490/85 and 189491/85). In this case, the microcapsule is partially broken by application of heat. It is expected, in this case, that the amount of energy required can be decreased as compared with the above method. At the same time, however, a problem arises with the storage stability of the microcapsules.
In order to utilize the permeability of the microcapsules, U.S. Pat. No. 4,579,770 discloses microcapsules incorporating a sublimating dye, and Japanese patent application (OPI) No. 196294/84 discloses microcapsules having the walls of Nylon and a synthetic bimolecular membrane. In the former, dye vapor resulting from sublimation of the dye permeates through the microcapsule wall, and thus a larger amount of energy is needed than in the general sublimation type heat transfer. In the latter, only a water-soluble colorant can be incorporated in the microcapsule owing to the structure of the bimolecular membrane, and the transfer image formed is poor in water resistance.
As described above, conventional melt type and sublimation type heat transfer recording techniques have advantages and disadvantages. That is, in the melt type heat transfer recording, gradation is difficult to obtain and also the transfer of an image to rough surfaces is difficult, and in the sublimation type heat transfer recording, energy sensitivity is low, transfer to ordinary paper cannot be attained, and the storage stability of images is poor.